Gordon Brown will this week pledge more Draconian powers to detain terror suspects without trial if he becomes prime minister, thrusting civil liberties to the heart of the battle for the Labour succession.

The Chancellor's move, on the eve of the anniversary of the 7 July bombings, marks a dramatic escalation in his campaign to prove he would be as tough on terrorism as Tony Blair. But his call for suspects to be detained without trial for more than 28 days - a limit controversially extended only eight months ago - will enrage Labour MPs who hoped he would be more libertarian than Blair, and it is already prompting Tory charges that he is playing politics with terrorism.

The pledge comes ahead of a potentially embarrassing report from an all-party committee, warning that Iraq has become a training ground for al-Qaeda with European recruits gaining 'field experience' before returning home. It calls on the government to publish terms under which it would pull troops out.

And it follows warnings from a senior minister that Labour will lose the next election if it cannot stop its infighting. Alistair Darling blamed Labour's by-election defeat in Blaenau Gwent on 'a number of difficulties' - from rows over school reforms to the allegations of peerages for sale - adding: 'I remain very optimistic that we will win the next election, but we are not going to do that if we cannot make sure that we are seen to be doing what the voters want us to. We need to spend less time contemplating internal affairs and more on what we are supposed to be doing.'

In such a febrile atmosphere, Brown's pronouncement will be carefully scrutinised. He will offer further safeguards over detentions before the period could be extended, including an independent parliamentary investigator with powers to examine any case in which new powers are used. He is understood to favour a gradual raising of the limit, perhaps by only a week or two at the time, in consultation with the public - and no move to change the law until Blair has left Downing Street.

However the Tories said, if there was an urgent case for change, then it should not wait until Brown was leader. 'It's an outrageous proposal: so far we haven't had a single use of the 28-day proposal, so to seek to extend it is nothing more than playing politics with terrorism,' said David Davis, the shadow Home Secretary. 'To raise this issue in the run-up to the anniversary of 7 July is insensitive and unwise.'

Brown will also announce a £40m cash boost this week for the security services. But a source close to him said: 'While this increased investment is important, the security forces also need tougher powers.' Brown will be backed up by a report from the Commons home affairs select committee, which will tomorrow condemn the way the government rushed to back police plans for 90 days' detention, but admit there are grounds to support detention beyond 28 days.

Peter Kilfoyle, a leading rebel on the last terrorism bill, said it showed Brown was simply 'the other side of the same coin' as Blair: 'They are going to repeat the same mistakes.'

Critics will also highlight the impact on the Muslim community. This week Sadiq Khan, Labour MP for Tooting in south London, will publish a Fabian Society pamphlet backing a public inquiry into the bombings and warning that Muslim task forces set up by the Home Office in the wake of 7/7 to advise on the way forward have been ignored, with only three of their recommendations implemented.